1. Technical Field of the Invention
This invention pertains to clipping circuits and, in particular, to audio clipping circuits used to limit instantaneous deviation of the carrier in a frequency modulated (FM) radio transmitter.
2. Description of the Prior Art
In an FM system, the occupied spectrum of the transmitted signal depends on the peak deviation of the carrier and the frequency content of the modulating signal. In voice applications, modulation levels can vary widely, and signal peaks often greatly exceed average modulation levels. For efficient communication, it is desirable to modulate at a high level; however, to conserve radio spectrum, it is desirable to limit peak deviation.
To avoid over-deviating the carrier while modulating at high average level with varying voice signals, FM transmitters often use clippers to limit voice modulation peaks. FM systems also pre-emphasize the modulation signals, both to allow for de-emphasis in FM receivers, which shapes the detected noise and improves recovered signal to noise ratio, and to compensate for thenatural high frequency roll-off of voice signals. Pre-emphasis before clipping allows more efficient communication by raising the level of the high frequency voice components and providing more total modulation energy for a given peak deviation.
FIG. 1a shows a block diagram of an FM radio transmitter that incorporates pre-emphasis and clipping as part of its modulation section. Voice signals enter port 2, receive amplification and bandlimiting in circuit 4, and go to an instantaneous deviation control (IDC) circuit 6, which provides pre-emphasis (8) and audio clipping (10). A bandlimiting filter (12) removes high frequency components caused by clipping; then the modulation signal capacitively couples (14) to the modulator and RF stages (16).
In practice, FM transmitters generally do not extend modulation response down to zero frequency (DC). This makes control of the carrier frequency easier, because a non-zero DC level in the modulation path does not affect the modulator. A non-zero DC level may arise from circuit offsets or from clipping of asymmetrical modulation signals. The waveforms in FIG. 1b, which depict signals at various points before and after the clipper of FIG. 1a, illustrate how this happens.
Waveform 30, which refers to signal 20 at the input to the clipper, can be asymmetrical for a number of reasons. For example, the stages preceding the IDC network 6 may have unequal dynamic ranges for positive and negative excursions from their operating points and may produce asymmetrical waveforms when overdriven. Except for unavoidable DC offsets, signal 20 would have zero average value, because pre-emphasis gives it a high-pass characteristic and removes its DC component. As shown by waveform 32, which refers to signal 22 at the output of the clipper, the clipper passes the portion of the signal within the clipping levels indicated as V.sub.h and V.sub.l. The input signal has zero average; however, because the clipper removes part of the positive peak, the output signal averages to negative V.sub.avg (33).
After bandlimiting in filter 12, the clipped signal couples to the modulator and appears at point 26. The modulator does not respond to DC; therefore, it responds to deviations about the average value of the signal. Waveform 36 shows the signal shifted positively by the amount V.sub.avg (33), so that it centers about its average, as the modulator would respond to it. The positive peak swings too far when measured relative to the average. This signal would cause the carrier to over-deviate in the direction corresponding to positive modulation.
To prevent over-deviation, the signal at point 22 should be clipped so that its peaks stay between V.sub.h and V.sub.l, measured relative to the average of the clipped signal rather than relative to zero. Clipping can change the average value of the signal, and the excursion from the average to one peak will be too large. If the signal were to be clipped and still have zero average, the problem would be eliminated. Waveform 36' shows an example of this. The signal has been altered so that it averages to zero yet stays within the clipping limits.
Some prior art circuit designs accomplish limiting by the use of high-gain, AC coupled, feedback amplifiers that saturate at predictable voltage limits. For example, see U.S. Pat. No. 4,491,972, issued Jan. 1, 1985, to Weber and assigned to the same assignee as this application. These limiters may incidentally achieve zero average output voltage in the steady state by using long time-constant coupling circuits, but they respond slowly to changes and allow transient overdeviation. Furthermore, to achieve miniaturization, it is frequently desirable to implement these limiters in monolithic switched capacitor form, but the long time-constants require large capacitor ratios, which are difficult and uneconomical to achieve.